Extended under-bump metal layer for blocking alpha particles in a semiconductor device

ABSTRACT

An integrated circuit (IC) has an under-bump metal (UBM) pad disposed between a solder bump and a semiconductor portion of the IC. The UBM pad has a contact perimeter formed with the solder bump. The UBM pad extends beyond the contact perimeter a sufficient distance to block alpha particles emitted from the surface of the solder bump from causing an upset event in the semiconductor portion.

FIELD OF THE INVENTION

This invention relates generally to techniques for fabricatingintegrated circuits, and more particularly to techniques for reducingsingle-event upset errors from solder bumps or balls.

BACKGROUND

Integrated circuits (ICs) using CMOS techniques are susceptible tosingle-event upsets due to alpha particles. An alpha particle can causeionizing radiation when it passes through a semiconductor device. Theresulting charge generated by an alpha particle can accumulate at adevice node and change the state of the node, typically by shorting atransistor source-drain and hence disturbing the logic state of thattransistor. For example, if a node in a memory cell stores a zero datavalue, charge accumulation at the node can flip the data value to a one.Such data state changes are commonly called “single-event upsets”(SEUs).

A common source of alpha particles is the material used in fabricatingball grid arrays (BGAs) or bump arrays on integrated circuits. Bumparrays and BGAs are often used to provide electrical and mechanicalconnections between an IC chip and a printed wiring board or packagecarrier. The material used to form the balls or bumps often containslead, which can be a source of alpha particles.

Various techniques have been developed to avoid alpha particles emittedby lead in solder bumps from creating SEUs in the associated IC. Oneapproach is to coat the solder bumps with a layer of alpha particleabsorbing material. Another approach incorporates a high-density layerof alpha particle absorbing metal deposited on selected areas of the IC.Both of these approaches introduce additional process steps into the ICmanufacturing sequence and hence add additional undesired cost.

Techniques that reduce SEUs due to alpha particles from balls or bumpson ICs that avoid the disadvantages of the prior art are desirable.

SUMMARY

An IC has a solder bump on an under-bump metal (UBM) pad disposedbetween the solder bump and a semiconductor portion of the IC. The UBMpad has contact perimeter formed with the solder bump, and extendsextending beyond the contact perimeter a sufficient distance to blockalpha particles emitted from the surface of the solder bump from causingan upset event in the semiconductor portion. In a further embodiment,underfill material surrounds solder bump. In a particular embodiment,the UBM pad extends at least ten microns from the contact perimeter. Inan alternative embodiment, the UBM pad is polygonal, and has a cornerdistal from the contact perimeter that is at least ten microns from thesurface of the solder bump.

In a particular embodiment, the UBM pad is fabricated from a UBM layercomprising copper at least nine microns thick. In a further embodiment,a UBM field is fabricated from the UBM layer. The UBM field is separatedfrom the UBM pad by a gap extending from the UBM pad to the UBM field soas to electrically isolate the UBM field from the UBM pad. The UBM fieldmay be left floating, or alternatively be connected to a voltagereference, such as ground or Vdd. In a particular embodiment, the IC hasa die surface area and a plurality of UBM pads. The combined area of theplurality of UBM pads and the UBM field underlie at least 99.9% of thedie surface area between a solder bump array and the silicon portion ofthe IC.

In a particular embodiment, the UBM pad extends over a passivation layerand a protective layer of the IC. A solder mask layer on the UBM paddefines a contact aperture having the contact perimeter. Alternatively,the UBM pad extends over a passivation layer and beneath a protectivelayer, which has been patterned to define a contact aperture having thecontact perimeter. The protective layer is, for example, a polyimidelayer. UBM pads are circular or polygonal, and may be of mixed shape orsize on an IC.

In an embodiment, an IC is fabricated by establishing a contact diameterof a solder bump for the IC. The IC has a BEOL stack, for which the BEOLalpha particle stoppage probability is calculated or otherwisedetermined. An underfill thickness having an underfill alpha particlestoppage probability is calculated so that the sum of the BEOL alphaparticle stoppage probability and the underfill alpha particle stoppageprobability provides a greater than 99% probability that an alphaparticle not greater than 5 MeV will be stopped by underfill and theBEOL stack. A UBM pad dimension is calculated to provide a UBM pad thatextends sufficiently beyond the solder bump contact diameter so that analpha particle emitted by the solder bump formed on a solder bumpcontact will travel through underfill from any point on the surface ofthe solder bump by at least the selected underfill thickness beforereaching the outer edge of the UBM pad when the IC is incorporated intoan assembly with underfill. The UBM pad is fabricated on the IC from aUBM layer to have the calculated UBM pad dimension and a solder bump isformed on the UBM pad. In a further embodiment, the IC is attached to acarrier (e.g., a package substrate or printed wiring board), andunderfill is applied between the IC and the carrier.

In a further embodiment, the UBM pad is separated from a field of UBMlayer material by a gap. In a particular embodiment, a plurality of UBMpads and the field is formed from the UBM layer. Each of the pluralityof UBM pads is isolated from the field by a corresponding plurality ofgaps. The plurality of UBM pads in combination with the field provide aremaining UBM layer area at least 99% of the IC die area. In aparticular embodiment, the UBM layer comprises a layer of copper orcopper alloy. In a more particular embodiment, the UBM layer is at least9 microns thick.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a solder bump and associated pattern of SEUsassociated with a conventional under-bump metal pad of an IC.

FIG. 2 is a side view of a solder bump and under-bump metal padaccording to an embodiment.

FIG. 3 is a plot of the energy blocked by passivation and polyimidelayers on an IC versus angle of incidence of alpha particles from asolder bump.

FIG. 4 is a side view representation of the penetration of 5 MeV alphaparticles in a top stack of an IC according to an embodiment.

FIG. 5 is a cross section of a solder bump on a UBM pad of an ICaccording to an embodiment.

FIG. 6 is a cross section of a UBM pad according to an embodiment.

FIG. 7 is a cross section of a UBM pad according to an alternativeembodiment.

FIG. 8 is a plan view of a UBM layout according to an embodiment.

FIG. 9 is a plan view of a portion of an IC wafer having an FPGA with abump array and under-bump metal layer according to an embodiment.

FIG. 10 is a flow chart of a method of fabricating an IC according to anembodiment.

DETAILED DESCRIPTION

FIG. 1 is a side view of a solder bump 100 and associated pattern ofSEUs associated with a conventional under-bump metal (UBM) pad 102 of anIC 103. A conventional UBM pad is basically a flat layer of metal, suchas a copper-based alloy or copper-nickel, that the bump material(typically a lead or lead-tin alloy, commonly referred to as solder)adheres to. The UBM pad is typically self-aligned to the bump edge. Forexample, bump material deposited on the IC wafer is heated to melt thebump material, which wets the UBM pad and forms a self-aligning ball orbump on top of the pad. For convenience of discussion, the term “bump”will be used to denote what are commonly called solder balls or solderbumps.

Alpha particles 104, 106 emitted from the lead in the bump 100 near theedges of the bump can reach the underlying transistors 105, such asstorage transistors in memory (e.g., SRAM) cells and cause analpha-upset event (e.g., corrupt the SRAM cell state). Since this typeof data upset is caused by a single event (i.e., a single alphaparticle), these events are called SEUs. An annulus (donut ring) of SEUs108 may be observed around solder bumps on an IC, particularly whereSRAM cells underlie the solder bump. The region of SEUs 108 is typicallyabout ten microns to about twenty microns from the inner diameter of theannulus to the outer diameter.

In particular ICs, alpha particles emitted from the periphery of thesolder bump (peripheral alphas) only cause SEUs in an annulus around theUBM pad 102. Underfill 112 adjoining the bump 100 is added after the ICis attached to the printed wiring board or carrier (not shown).Underfill processes are commonly known in the art of IC assembly, andepoxy or epoxy composites are common underfill materials. The underfill112, in combination with the IC passivation layer 114 and masking orprotective layer 116, which in a particular IC is a layer of polyimidematerial about seven microns thick, block alpha particles 110originating more than about ten microns from the surface 118 of the ICdie.

The angle of incidence is measured from the normal direction to themajor surface of the IC die. In other words, an alpha particle hittingthe surface perpendicularly has an angle of incidence of zero degrees,while an alpha particle at thirty degrees from normal (i.e., sixtydegrees from the planar surface) has an angle of incidence of thirtydegrees. As the angle of incidence increases, the distance that thealpha particles traverse through the underfill 112, protective layer116, and passivation layer 114 increases. For example, at an angle ofincidence of forty-five degrees, the distance traversed by an alphaparticle originating at the periphery of the solder bump increases by afactor of 1.41 times. Extending the UBM pad beyond the perimeter of thesolder bump can block peripheral alphas from reaching the underlyingtransistors and memory cells, and reduce SEUs.

FIG. 2 is a side view of a solder bump 100 and UBM pad 200 according toan embodiment. The UBM pad 200 extends a selected distance beyond thecontact perimeter 202 of the solder bump 100 with the UBM pad 200 and issurrounded by underfill 112. In a particular embodiment, the UBM pad 200is fabricated from a copper-nickel layer at least about nine micronsthick, which is sufficiently thick to block alpha particles having anenergy of 5 MeV or less. Such alpha particles are similarly blocked ifthey that have traveled through at least ten microns of epoxy underfilland BEOL stack. The BEOL stack is the layers of patterned metal andintervening dielectric material of an IC. A complex IC, such as afield-programmable gate array (FPGA), may have ten or more patternedmetal layers separated by inter-metal dielectric layers in a BEOL stack.The BEOL stack in combination with the passivation and polyimide layerswill be referred to as the top stack of the IC for purposes ofdiscussion.

In a particular embodiment, the UBM pad 200 extends at least about tenmicrons beyond the perimeter 202 of the solder bump 100, which is about110 microns at the widest point 204 (essentially a sphere with adiameter of 110 microns that is truncated on the bottom, where it isbonded to the pad). Alternative extended distances are used with othersized solder bumps and balls, as the overhang of the solder bump fromthe contact area of the pad changes with bump diameter. In anotherembodiment, the UBM pad 200 is extended about twenty microns to aboutthirty microns beyond the perimeter of the solder bump 100. While alphaparticles 210 have a distribution of energies, extended UBM padsaccording to embodiments essentially block alpha particles emitted fromsolder bumps from upsetting underlying memory cells or transistors 205,207. A passivation layer 114 (e.g., a silicon nitride layer about 0.5microns thick) and a protective layer 116 (e.g., a polyimide layer aboutseven microns thick) overlie patterned metal layers alternating withdielectric layers 209, which provide electrical interconnections in theIC.

FIG. 3 is a plot 300 of the energy blocked by passivation and polyimidelayers on an IC in combination with the BEOL stack (see, FIG. 1, ref.nums. 114, 116) versus angle of incidence of alpha particles from asolder bump for an exemplary FPGA. The natural decay chain for Pb cancreate alpha particles up to about 6.5 MeV, but the vast majority ofalpha particles being emitted from a solder bump have an energy lessthan about 5 MeV. Alpha particles having energy of about 5 MeV or lesshitting the top stack at an angle of forty-five degrees or more areblocked by the top stack layers. UBM pads that extend sufficiently toblock peripheral alpha particles with an angle of incidence less thanforty-five degrees, in cooperation with the top stack layers, blockessentially all of the peripheral alpha particles.

FIG. 4 is a side view representation of the penetration of 5 MeV alphaparticles 400 in a top stack of an IC according to an embodiment. Thepenetration of the alpha particles was modeled using techniquesaccording to those developed by J. F. Ziegler for determining thestopping and range of ions in matter (SRIM techniques). Therepresentation is for an alpha particle at an angle of incidence of zerodegrees, which is normal to the BEOL stack layers of an IC. Beforereaching an underlying silicon device, an alpha particle passing throughabout ten microns of epoxy 402, about seven microns of polyimide 404,about 0.5 microns of a silicon nitride 406, about 1.4 microns ofaluminum (e.g., upper layer wiring) 408, about 2.7 microns of silicondioxide 410, and about one micron of copper 412 would be stopped beforereaching the underlying silicon. Material stopping powers vary withelemental composition, and copper has much higher stopping power thanpolyimide or silicon dioxide per unit thickness.

The thicknesses of silicon dioxide and copper represent the cumulativethicknesses of these materials in a typical BEOL stack of patternedmetal layers separated by inter-metal dielectric (IMD) layers. Eachpatterned metal layer has an associated density (i.e., the portion ofthe total area that is metalized), which in a typical metal layer isabout 50% to about 60%. The probability that an alpha particle would hita copper trace in a particular metal layer corresponds to the metallayer density. The thickness of copper in FIG. 4 basically accounts forthe layer densities of the multiple metal layers (e.g., a ten metallayer stack), and represents a cumulative probability that an alphaparticle would encounter multiple copper traces in multiple metallayers, but not necessarily a copper trace in each metal layer. An alphaparticle having an energy of 5 MeV would have a very high probability ofbeing stopped by the combination of the epoxy, polyimide, and BEOLstack, and would not reach the underlying silicon 414.

Prior ICs were often built with larger dimensions (commonly referred toas node geometry). ICs built on larger node geometries generally havethicker layers, wider traces, and so forth. In large geometry ICs havingmany patterned metal layers in the BEOL stack, the cumulative thicknessof the metal traces combined with the probability that an alpha particlewould hit multiple, relatively thick, copper traces as it traveledthrough the BEOL stack prevented many alpha particles from reaching theunderlying silicon. SEUs arising from solder bump alpha emissions wererelatively low in large geometry devices. As dimensions have beenreduced, the thicknesses of the layers (and hence the thickness of thecopper traces in the patterned metal layers of the BEOL stack) has alsobecome reduced. The effect of having thinner copper traces in thepatterned metal layers is that more alpha particles were able to reachthe underlying silicon, producing the ring of SEUs discussed inreference to FIG. 1. Thus, embodiments of the invention are particularlydesirable in ICs fabricated on a node geometry of 90 nm or less, andmore particularly desirable in ICs fabricated on a node geometry of 40nm.

FIG. 5 is a cross section of a solder bump 502 on a UBM pad 504 of an IC500 according to an embodiment. The solder bump 502 is formed on a UBMpad 504 that is masked with a layer of masking material 506 such assilicon dioxide, silicon nitride, polyimide or other suitable maskingmaterial. Contact apertures are defined in the masking material, and thesolder bumps are formed in a self-aligned fashion over the contactapertures to bond with the underlying UBM pad 504. The solder bump has adiameter 508 of about 110 microns; however, this dimension is merelyexemplary and for purposes of illustration.

The UBM pad 504 is a copper-nickel alloy or other suitable metal forforming solder bumps on, and in a particular embodiment is about ninemicrons thick. The UBM pad 504 extends at least about ten microns fromthe perimeter 510 of the contact aperture. The extended UBM pad 504blocks alpha particles 512, 514 emitted from the periphery of the solderbump 502 from upsetting underlying transistors or memory cells 516 inthe silicon 518 below the solder bump 502. An alpha particle originatingmore than ten microns above the BEOL stack 526 is blocked by the epoxyunderfill 528 and the top stack 526. Similarly, an alpha particle 530originating at an angle of incidence sufficient to clear the corner 534of the UBM pad 504 is blocked by the underfill 528 and top stack 526.Thus, extending the UBM pad 504 beyond the contact aperture according toembodiments blocks alpha particles emitted from the periphery of thesolder bump 502 from reaching the underlying transistors and SRAM cells,reducing SEUs. The IC is attached to a carrier (not shown), such as apackage substrate or printed wiring board. After attaching the IC (e.g.,through a solder re-flow process), underfill 528 is applied, as is knownin the art of circuit assembly using BGA techniques.

FIG. 6 is a cross section of an UBM pad 600 according to an embodiment.A solder bump (not shown) provides electrical contact to a conductivetrace (wire) 602 of an IC 604. The conductive trace 602 is defined in atop metal layer, for example, and is an aluminum trace in a particularembodiment. A UBM pad 600, which is defined from a UBM layer of copper,copper-nickel, or other metal about 9 microns thick, is defined over theconductive trace 602, which it makes electrical contact to, and over aprotective layer 608, which in a particular embodiment is a layer ofpolyimide about 7 microns thick. A passivation layer 610, which in aparticular embodiment is a layer of silicon nitride about 0.5 micronsthick, underlies the protective layer 608. A bump masking layer 612defines a bump aperture 614 of the UBM pad 600. The UBM pad 600 extendsa selected distance 616 beyond the perimeter edge 618 of the bumpaperture 614 to block peripheral alpha particles emitted by the solderbump (not shown), which typically forms an approximate truncated sphereon the exposed portion (i.e., within the aperture) of the UBM pad 600.In a particular embodiment, the UBM pad 600 extends at least about tenmicrons beyond the perimeter edge 618 of the bump aperture.

FIG. 7 is a plan view of an under-bump metal pad 700 according to analternative embodiment. A solder bump (not shown) provides electricalcontact to a conductive trace (wire) 602 of an IC 704. The conductivetrace 602 is defined in a top metal layer, for example, and is analuminum trace in a particular embodiment. The UBM pad 700 is definedfrom a UBM layer of copper, copper-nickel, or other metal about 9microns thick, is defined over the conductive trace 602, which it makeselectrical contact to, and over a passivation layer 710, which in aparticular embodiment is a layer of silicon nitride about 0.5 micronsthick. Instead of using a separate solder mask layer (compare FIG. 6,ref. num. 612), a polyimide layer 708, which in a particular embodimentis about 7 microns thick, is patterned to form an aperture 714. The UBMpad 700 extends a selected distance 716 beyond the perimeter edge 718 ofthe bump aperture 714 to block peripheral alpha particles emitted by thesolder bump (not shown), which typically forms an approximate truncatedsphere on the exposed portion (i.e., within the aperture) of the UBM pad700. In a particular embodiment, the UBM pad 700 extends at least aboutten microns beyond the perimeter edge 718 of the bump aperture.

FIG. 8 is a plan view of a UBM layout according to an embodiment. UBMpads can be circular or polygonal, and solder bump contact apertures canbe circular or polygonal. The UBM layer has a UBM pad 804 separated fromthe field of the UBM layer 806 by a gap 810 that provides electricalisolation between the field 806 and the pad 804. A solder bump 802 isgreater in diameter than a contact aperture (not shown, see, e.g., FIG.6, ref. num. 614, FIG. 7, ref. num. 714). For embodiments utilizing apolygonal UBM pad, it is desirable that the edge of the UBM pad 804extend beyond the widest point of the solder bump 802 a selecteddistance 812 sufficient to block peripheral alpha particles from thesolder bump 802 from causing an SEU in the underlying semiconductordevice.

The UBM field 806 blocks alpha particles originating from underfillmaterials (not shown) or elsewhere. The gaps 810 are a minor portion ofthe area of the UBM layer, and in a particular embodiment constituteless than 0.1% of the die area (i.e., the plurality of UBM pads and theUBM field underlies at least 99.9% of the die surface area). Combinedwith lower count for underfill, such as ultra-low-alpha (ULA) underfill,which is specified to have less than 0.002 counts per hour/cm², SEUsarising from solder bump and underfill can be reduced to less than1/1000^(th) from IC assemblies using conventional UBM layouts. In aparticular embodiment, the gaps are approximately one micron between theUBM pads and the UBM field. Embodiments in accordance with FIG. 8 areparticularly desirable in ICs fabricated on small node geometries (e.g.,less than about 90 nm node geometry), or ICs having a relatively lownumber of metal layers in the BEOL stack, or ICs having low densitymetal layers in the BEOL stack, as such conditions diminish the alphaparticle blocking effectiveness of the BEOL field.

FIG. 9 is a plan view of a portion of an IC wafer 900 having an FPGA 902with a bump array and under-bump metal layer according to an embodiment.The FPGA 902 is separated from other FPGAs on the IC wafer (e.g., FPGA904) by alleys 903. ICs 902, 904 will be singulated from the IC wafer900 for use in applications. An IC chip with a bump array is typicallyflip-chip bonded to a printed wiring board or package substrate.Underfill (not shown in this view) is typically applied between the ICchip and the wiring board or package substrate.

Solder bumps 906, 908 are formed on UBM pads (not shown in this view)that extend beyond the contact aperture of the solder bumps 906, 908 toblock peripheral alpha particles emitted by the solder bumps 906, 908.In a further embodiment, a UBM field (see, e.g., FIG. 8, ref. num. 806)underlies most of the area of the IC 902 and blocks alpha particles fromother sources. The UBM field is separated from the UBM pads by gaps(see, e.g., FIG. 8, ref. num 810), which electrically isolate the UBMpads from the UBM field. The UBM layer remaining after the IC isfabricated (i.e., the UBM pads and field combined) is at least 99% of ICdie area, thus shielding at least 99% of the IC from alpha particleshaving energy less than the energy required to penetrate the UBM layer.

FIG. 10 is a flow chart of a method of fabricating an integrated circuit1000 according to an embodiment. A solder bump contact diameter isestablished (e.g., specified) for an IC having a BEOL stack (step 1002).A BEOL alpha particle stoppage probability is determined for the BEOLstack (step 1004) (see, e.g., FIG. 4 and associated WrittenDescription). An underfill thickness having an underfill alpha particlestoppage probability is calculated so that the sum of the BEOL alphaparticle stoppage probability and the underfill alpha particle stoppageprobability provides a greater than 99% probability that an alphaparticle not greater than 5 MeV will be stopped by the underfill andBEOL stack (step 1006). A UBM pad dimension is calculated sufficient toextend the UBM pad beyond the solder bump contact diameter so that analpha particle emitted by a solder bump formed on the solder bumpcontact will travel through underfill by at least the selected underfillthickness before reaching the outer diameter of the UBM pad from anypoint on the surface of the solder bump when the IC is attached to (step1008). The UBM pad is fabricated on the IC from a UBM layer to have thecalculated UBM pad dimension (step 1010). A solder bump is formed on theUBM pad contacting the UBM pad at a contact diameter (step 1012). In afurther embodiment, the IC is attached to a carrier (e.g., a packagesubstrate or printed wiring board) (step 1014), and underfill is appliedbetween the IC and the carrier (step 1016).

In a particular embodiment, the UBM layer is a layer of copper or copperalloy. In a more particular embodiment, the UBM layer is about 9 micronsthick.

In a further embodiment, before fabricating the UBM pad, a gap isdefined in the UBM layer between the UBM pad and a field of the UBMlayer. In a particular embodiment, the field and pads of the UBM layerare designed to cover over 99% of the IC, which blocks alpha particlesarising from the underfill material or other sources, and blocks most ofthe alpha particles emitted from the solder bump in assembliesincorporating small node geometry ICs that do not use underfill, andhence would not achieve the alpha particle blocking contribution of theunderfill of step 1006, above.

While the present invention has been described in connection withspecific embodiments, variations of these embodiments will be obvious tothose of ordinary skill in the art. For example, an IC with a UBM layerfield may be used without underfill, or alternative BEOL stacks may beincorporated in the IC. Therefore, the spirit and scope of the appendedclaims should not be limited to the foregoing description.

1. An integrated circuit (IC), comprising: a solder bump having asurface; a semiconductor portion, and an under-bump metal (UBM) paddisposed between the solder bump and the semiconductor portion, the UBMpad having a contact perimeter formed with the solder bump, wherein theUBM pad extends beyond the contact perimeter a sufficient distance toblock alpha particles emitted from the surface of the solder bump fromcausing an upset event in the semiconductor portion.
 2. The IC of claim1, further comprising: underfill material disposed around the solderbump.
 3. The IC of claim 2, wherein the UBM pad extends at least tenmicrons from the contact perimeter.
 4. The IC of claim 2, wherein theUBM pad has a corner distal from the contact perimeter, the corner beingat least ten microns from the surface of the solder bump.
 5. The IC ofclaim 1, wherein the UBM pad is fabricated from a UBM layer comprisingcopper, the UBM layer being at least nine microns thick.
 6. The IC ofclaim 5, wherein: the IC further comprises a UBM field fabricated fromthe UBM layer; and the UBM field is separated from the UBM pad by a gapextending from the UBM pad to the UBM field so as to electricallyisolate the UBM field from the UBM pad.
 7. The IC of claim 5, wherein:the IC has a die surface area and a plurality of UBM pads; and theplurality of UBM pads and the UBM field underlie at least 99.9% of thedie surface area.
 8. The IC of claim 1, wherein: the UBM pad extendsover a passivation layer and a protective layer of the IC; and a soldermask layer on the UBM pad defines a contact aperture having the contactperimeter.
 9. The IC of claim 1, wherein: the UBM pads extends over apassivation layer and beneath a protective layer; and the protectivelayer is patterned to define a contact aperture having the contactperimeter.
 10. The IC of claim 9, wherein the protective layer is apolyimide layer.
 11. The IC of claim 1, wherein the UBM pad is circular.12. The IC of claim 1, wherein the UBM pad is polygonal.
 13. A method offabricating an integrated circuit, comprising: establishing a contactdiameter of a solder bump for an IC having a BEOL stack; determining aBEOL alpha particle stoppage probability for the BEOL stack; calculatingan underfill thickness having an underfill alpha particle stoppageprobability so that the sum of the BEOL alpha particle stoppageprobability and the underfill alpha particle stoppage probabilityprovides a greater than 99% probability that an alpha particle notgreater than 5 MeV will be stopped by underfill and the BEOL stack;calculating a UBM pad dimension sufficient to extend the UBM pad beyondthe solder bump contact diameter so that an alpha particle emitted bythe solder bump formed on a solder bump contact will travel throughunderfill from any point on the surface of the solder bump by at leastthe selected underfill thickness before reaching the outer edge of theUBM pad when the IC is incorporated into an assembly with underfill;fabricating the UBM pad on the IC from a UBM layer to have thecalculated UBM pad dimension; and forming the solder bump on the UBMpad.
 14. The method of claim 13, further comprising attaching the IC toa carrier; and applying underfill between the IC and the carrier. 15.The method of claim 13, wherein fabricating the UBM pad from the UBMlayer includes fabricating a gap between the UBM pad and a field in theUBM layer.
 16. The method of claim 13, wherein: fabricating the UBM padincludes forming a plurality of UBM pads and a field of the UBM layer;each of the plurality of UBM pads is electrically isolated from thefield by a corresponding plurality of gaps; the IC has an IC die area;and the plurality of UBM pads in combination with the field comprise aremaining UBM layer area at least 99% of the IC die area.
 17. The methodof claim 13, wherein the UBM layer comprises a layer of copper or copperalloy.
 18. The method of claim 17, wherein the layer of copper or copperalloy is at least 9 microns thick.